Preserve whitespace when serializing the DOM as HTML

[westonruter]

Add workaround to prevent PHP from adding whitespace to format the serialization of HTML. This prevents bugs related to spaces being added between elements which can break layouts. For example, without this fix, the following HTML:

<style>
.cols-test > li {
    width: 50%;
    display: inline-block;
    outline: solid 1px red;
}
</style>
<ul class="cols-test"><li>column 1</li><li>column 2</li></ul>

Which should get rendered as:

Will get serialized instead as:

<ul class="cols-test">
<li>column 1</li>
<li>column 2</li>
</ul>

And displayed as (with text selected to show whitespace insertion):

For some more background on the changes here, see https://stackoverflow.com/q/51660286/93579

Fixes #1304.

[westonruter]

Note that to test, do not use the <ul> example in post content. There is a the_content filter which adds line breaks for that. Instead try the markup originally reported in #1304.

[westonruter]

For anyone who wants, here is a build to test: amp.zip (v1.0-beta1-95dc4069-20180802T220041Z)

To test, deactivate and uninstall the existing AMP plugin and then go to the plugins admin page and click Upload Plugin:

Then select the amp.zip file to upload and then activate.

Please test this on a non-production environment.

[kienstra]

Approved

Hi @westonruter,
This looks good. Using your code example, I also saw that this PR fixed the issue of spaces being inserted between tags.

Before this PR:

With this PR:

[Frenchomatic]

@westonruter Just to confirm in case you missed it in my post - this fixes it all for me both in your plugin and the AMP for WP one too which uses the utils folder in particular - thank you.

[Zabi09]

@westonruter
We really appreciate your hard work in fixing the spaces issue from the source code and we have tested the commits on PHP Version 7.2.5 and it was working perfectly,

Will you please tell us how to test the same with the 7.3 version of PHP and for your information we have downloaded the beta version of 7.3 version but unable to check it

Please let us know So that we can check it and update the code on our AMPforWP plugin

[westonruter]

@Zabi09 From what I can tell the issue is no longer present in PHP 7.3: https://3v4l.org/TS2tX

This being the case we can bypass the whole logic in:

https://github.com/Automattic/amp-wp/blob/95dc40692df5f3216d5b0ffa35136ab6f0b1383d/includes/utils/class-amp-dom-utils.php#L413-L414

I'll be updating the Travis build matrix to include PHP 7.3 once it is available: travis-ci/travis-ci#9717